The present invention relates to mixing apparatus and particularly to a mixing impeller which produces axial flow and also is effective in dispersing one fluid (either a liquid or gas) into another fluid as the fluids are mixed by the impeller. The mixing apparatus is especially suitable for use in sparging systems for mass transfer (the transfer of the mass of one fluid phase into another) and in stirred reactors or agitators where the other fluid phase is generated during the reaction, as for example in liquid-liquid dispersion processes.
The invention provides a mixing impeller which affords efficient axial flow in the direction of the axis about which the impeller rotates, while providing a shear field which reduces the size of the media being dispersed without a significant or practical impact upon the flow efficiency of the impeller.
Mixing impellers, which have been used for introduction of air or another fluid, operate effectively when the gas or other fluid is dispersed in the form of fine droplets. Such fine droplets make mass transfer or dissolving processes more efficient, for example, as measured by the mass transfer coefficient of the mixing system, kLa. The introduction of the gas or other fluid is called sparging. Axial flow impellers, particularly those with large blades, have been effectively used in sparging and provide high efficiency by benefiting from the efficient axial flow now produced by the impeller. One such axial flow sparging system, and mixing impellers used therein, are described in Weetman et al, U.S. Pat. No. 5,046,245 issued Sep. 10, 1991. It is desirable, therefore, to use axial flow impellers, and particularly those with even more efficient pumping as a obtained by narrow blades, such as described in Weetman U.S. Pat. No. 4,468,130 issued Aug. 28, 1984, which are even more efficient in terms of flow than the impellers described in U.S. Pat. No.5,046,245. Nevertheless, the efficiency of sparging is adversely affected by the size of the bubbles of the phase (liquid of gas or gas) being dispersed into the flow produced by the mixing impellers. It has been proposed to provide turbulence adjacent the impeller by various elements which disrupt the flow (See for example, Cooke 4,662,823, May 5, 1987 and Kozma et al, 5,312,567, May 17, 1994 and 5,431,860, Jul. 11, 1995). However, the turbulizing elements block the flow and sacrifice the inherent efficiency of the impeller.
It is a feature of the present invention to provide a shear field which effectively breaks down the size of the bubbles or droplets of the phase to be dispersed and disperses the phase in the form of much finer bubbles or droplets than would otherwise be the case. The invention provides structure carried by the impellers which creates the shear field. Such structure is to be distinguished from expedients to reduced turbulence and discontinuities in the flow about an around an impeller or to stabilize the impeller; for example, slotted or multiple blades or fins or other projections attached to the impeller blades. See in this connection the following U.S. Pat. Nos.: Zeides, 4,636,143, Jan. 13, 1987, Tomohiro et al, 4,893,990, Jan. 16, 1990, Mita 5,226,783, Jul. 13, 1993, Kato et al, 5, 277,550, Jan. 11, 1194, Weiss et al, 5,595,475, Jan. 21, 1997, Miura, 5,326,168, Jul. 5, 1994, Connolly et al, 5,525,269, Jun. 11, 1996, and also the proplets which control reverse flow around the tips in axial flow impellers such as described in the above referenced Weetman U.S. Pat. No. 4,468,130 and see also in Weetman U.S. Pat. No. 4,802,771, Feb. 7, 1989.
Accordingly, it is the principal object of the present invention to provide improved axial flow impeller apparatus which introduces shear for breaking up bubbles of a fluid phase while preserving axial flow efficiency of the impeller.
It is a still further object of the present invention to provide improved axial flow impellers which are especially suitable for use in sparging systems.
It is a still further object of the present invention to provide an improved axial flow impeller which shears droplets or bubbles into finer droplets or bubbles and disperses them into an axial flow stream produced by the impeller.
It is a still further object of the present invention to provide improved axial flow impellers which can be used in a tank in which one or more such impellers may be mounted for rotation on a shaft and where fluid is released into the tank in the form of fine bubbles or droplets which are mixed and dispersed throughout the liquid in the tank by the impeller or impellers.
Briefly described a mixing impeller provided in accordance with the invention is effective for dispersing a first fluid, that may differ in density or viscosity from a second fluid, into the second fluid while mixing the fluids with each other. The impeller has a plurality of blades oriented with respect to an axis about which the impeller rotates to provide axial flow of the fluids. The blades each have a structure providing a Venturi which is rotatable with the blades defining flow path having a width in a direction radially of the blades which is larger nearer the leading edge of the blades than the trailing edge thereof. The flow path thus converges in a direction from the leading edge to the trailing edge and may be wedge shaped. The Venturi creates a shear field which shears the second fluid into fine bubbles or droplets and disperses it into the first fluid during the mixing of the fluids by the impeller, thereby enhancing sparging and mass transfer processes which are facilitated by the mixing impeller.
The foregoing and other objects, features and advantages of the invention, as well as presently preferred embodiments thereof will become more apparent from a reading of the following description in connection with the accompanying drawings, brief descriptions of which follow.